var fs = require( 'fs' );
function pReadFile ( filePath ) { 
    return new Promise( function ( resolve, reject ) { 
        fs.readFile( filePath, 'utf8', function ( err, data ) { 
            if ( err ) {
                reject( err );
            } else { 
                resolve( data );
            }
        })
    })
}
pReadFile( './data/a.txt' )
    .then( function ( data ) { 
        return pReadFile('./data/b.txt')
    } )
    .then( function ( data ) { 
        return pReadFile( './data/c.txt' )
    })